Method and apparatus for flotation of hydrocarbon impurities



May 9, 1961 T. F. G. BOYD ET Al. 2,983,577

METHOD AND APPRATUS FOR FLOTATION OF HYDROCARBON IMPURITIES OriginalFiled Feb. 27, 1951 METHOD AND-APPARATUS FOR FLDTTION 0F HYDROCARBONIMPURITIES Thomas F. G. Boyd, Orcutt, Paul W. Fischer, Whittier, andVRobert T. Wheeler, Santa Maria, Calif., assignors to Union Oil. Companyof. California, Los Angeles, Calif., a corporation of CaliforniaOriginal application Feb. 27 1951, Ser. No.. 212,985;

now Patent No; 2,759,607,I dated- Aug.- 21, 1956-. D1- ,vided' and thisapplication Dec. 12, 1955, Ser. No.

12 Claims.A (Cl. Zul-44)- 15 This invention relates to a method forpurification of aqueous suspensions and emulsions, and particularly tochemicals and otation equipment for the purpose, and is` a division ofour copending application Serial No. 212,985, now U.S. Patent No.2,759,607.

Water which is contaminated with oil and/or solid material is frequentlymet in industry. For example, water produced with petroleum or used inthe drilling or production of oil wells is generally contaminated withoil as well as some solids, and is frequently in the form of a waterexternal emulsion which is very diicult to break. Emulsions andsuspensions also occur in 'Waste water from fish canning, fruit dryingand various other processes, pump packings, leaks, engine coolants, etc.In oil field practice for example, waste water from varous sources,which may be contaminated with. oil, isV generally sent to a separatingtank and frequently thereafter to a skimming pond where it is allowed tosettle. The oil is skimmed oli and utilized and the purified water isthen used for secondary recovery purposes or sent to the usual disposal,such as sewage lines. It is often the case however that the suspensionsand/or emulsions which are formed cannot be broken simply by settling,and that the water so recovered, even though apparently clear, stillcontains as much as 200 or 300 parts per million of oil, emulsiiied orsimply ydistributed through the water. It may also have or develop anoil filmor a turbidity, due to oil or solids. This means that the watercannot be dropped to sewage disposal, since there are generally maximumlimits imposed by the government on the oil and/or solids content ofwater entering sewer lines. The utilization of the water for otherpurposes, such as for injection into wells for secondary recovery of oilby water drive, is also seriously impaired by the presence ofappreciable amounts of oil and/or solid material, since these may tendto plug the formation.

It is an object of this invention to provide a method and an apparatusfor separating suspensions or emulsions which are diflcult to separateby other processes such as simple settling.

It is a further object of this invention to separate solid materialand/or oil from water which is contaminated with relatively smallamounts of these materials.

It is a further object of this invention to provide a flotationapparatus for separating oil and/or solids from Water.

It is a further object of this invention to provide a method fortreating emulsions or suspensions with chemicals to facilitate theirseparation in flotation processes.

It is a further object of this invention to purify oil iieldwaste Waterto make it suitable for disposal in sewers or the like, or for use insecondary recovery by water drive. v

Further objects of the invention will be apparent from the followingdescription of the invention.

Accordingtov the present invention, emulsions or suspensions of oiland/or solids and Water are thoroughly mixed with air orV otherflotation gas, preferably at a somewhat elevated pressure. The resultingemulsion containingentrained and dissolved gas is dischargedhorizontally through pressure reducing means if necessary into one endof .a otation unit which isV relatively narrow; tall, and long, atapoint just below the levelof the liquid therein, and at a relativelyhigh rate. A specially designed bae at the oppositel end of thevrflotation unit serves to circulate the water backrto the entrance endof the flotation unit at a lower level, where it is again cir,-

culated to the far end along the bottom ofthe tankV and passes behindthe bale and out of the tank over a weil` at the discharge end of theunit.

liberated continuously in the tank in theform of exceed'- ingly smallbubbles, which have been found to be extremely etective in the dotation.rThe oil and/or certain solids are oated by the bubbles and rise to thesurface from which they are skimmed near the dischargeend of. the unit.The heavy solids if any, separate readily rwhen the oil has beenseparated, and they fall to the bottom of the unit, and are Withdrawntherefrom directly.

The design of the equipment will become more clear by reference to theattached drawings in which I Figure l is a sectional side elevation ofthe flotation unit,

Figure 2 is an end elevation of the discharge end of the unit,

Figure 3 is a vertical section taken through 3-3 of Figure l, and

Figure 4 is an end elevation of the feed end of the unit. For greaterclarity, the paddle wheel is omitted. from Figures 2 to 4.

The same numbers, are employed to designate the same parts oi theequipment in all the drawings.

Referring `to thev drawings, the process will be described in connectionwith the purification of oilv lieldl Waste water, which is anoil-in-water emulsion which also contains some suspended solids. Wastewater feed enters line 1 at a high crate and passesV through valve 2intoV pump 3, where it is pressurized toa pressure of 40 to 5 0 poundsper square inch, and remains at substantially thispressure on itsjourney through line 4. On passing through valve 5, the pressure isreduced tov substantially ratmospltreric pressure, and the water ispassed through line 6 into flotation line 7. Air under higher pressureis introduced into line 4 through line 8 and valve- 9 also.

at a relatively high rate. The air is preferably introduced close to thepump discharge, so that the waste 'Water becomes very thoroughlycontacted with air at Ithe ele.- vated pressure and great turbulence inthis line. icals may be introduced into the waste water feed eitherVinto line 1 through line 10a, or into line -6 through line 10 and valve11. Line 6 is preferably'split at its entry into cell, 7 into a numberof lines entering the unitat the same horizontal level and distributeduniformly across the width of the end. The same purpose may beaccomplished by discharging the mixture through a horizontal slit in alpipe laid across the end of the cell.

Line 6 enters cell 7 just below the liquid level 12 maintained therein,and discharges horizontally, so that there is a current created at thesurface from the inlet 6 to the baliles 13 and 14 at the far end of theunit. Bales 13 and 14 are curved and concave toward the inlet of thetank, primary baille 13 extending slightly above the liquid surface anddown to about the mid-point of the tank,

and secondary baille 14 extending from a point well above Theygas whichwas Y entrained and dissolved during thev mixing operation is Chem- 314, and the lower end of baiile 13 is attached to baille 14 at about itsmidpoint. This creates a trough 21 between baffle 13 Vand the upperportion of bafe 14.

The water circulating back from vbaffles 13 and 14 finally passes underthe lower end of baffleld and back of baffle 14 tothe surface. Itoverflows over Weir 15 into discharge box 16, from which it is removedthrough line4 17 and valve 18. Meanwhile the oil and any light solidswhich may have been caused to Afloat by the tiny air bubbles releasedfrom` the incoming water by the pressure reduction are skimmed from thesurface of the liquid in tank 7 by means of skimmer baffle 19 andpaddles 20, which are rotated by means not shown. The oil is thusskimmed from the surface and passes into `the trough 21 previouslydescribed, and is withdrawn therefrom through line 22 and valve 23. Thesolids which have settled from the water during the process arewithdrawn from the lowest point of lthe cell V7 through line 24 andvalve 25, in the form of a heavy aqueoussludge. Weir 1S is adjustablevertically by means of crank 26 and screw 27 which passes through frame29 and is attached to the Weir through bracket 28. The adjustment servesto control the liquid level 12 in the tank.

There are a number of features of the above apparatus which have beenfound to be critical. Thus the dimensions of tank 7 should be such thatit is relatively narrow and long. For example, if DI designates thedepth of the liquid from surface 12 down to the bottom of the tank atthe inlet end thereof; and L designates the length of the tank asmeasured from the inlet end to the baffles 13 and 14, the Width W shouldnot exceed about 0.2 to about 0.7 times DI and should also not exceedabout 0.1 to about 0.5 ltimes L. The trough between balfle 13 and theupper portion of baille 14 need only be large enough to accommodate theoil and foam skimmed olf the surface of the tank, and the space behindbale 14 need only be large enough to permit the clarified water to flowfreely over the weir and out of the tank.

' The point of feed inlet is quite important. It might be expected thatintroducing the feed at a considerable depth below the surface of thewater in the tank would provide a longer path for the air bubbles andincrease the efficiency of the flotation. However it has been found thatit is preferable to introduce the feed at a point very close to thesurface of the liquid. For example, the top of the inlet tube or tubesmay be at the liquid level or immersed not more than about 0.2 times thedepth of the tank at the inlet end thereof.

The primary baffle 13 should be placed so that its top extends slightlyabove the liquid level in the tank. Its bottom, i.e., where it joinsbaille 14, should be located at a distance Dp which is between about 0.3and 0.7 times DI the depth of the liquid at the inlet end of the tank.The radius of curvature RP of this baffle should be between about 0.7and 1.5 times lDp; and the center of the circle defining the radius ofcurvature should be located below the liquid level at a distance Dcwhich is between about 0.3 and 0.7 times Dp.

As to the position of the secondary baffle 14, the upper end extendsabove liquid level 12, the point at which it joins baille 13 is definedabove as DP, and the distance of its bottom below the liquid level, Dsshould be between about 0.6 and 0.95 times D0, the depth of the liquidat the location of the bottom of baffle 14. The radius of curvature RSof the secondary baille should be greater than that of the primarybaffle in general and should be between about 0.7 and 1.5 times DI thedepth of the liquid at the inlet end of the tank. The center of thecircle defining the radius of curvature should be located atapproximately the liquid level, or at a distance DC which is eitherabo-ve or below the liquid level not more than about 0.2 times DI.Although a curved shape for bale 14 is preferred, it may be straight orangular, providing that the lower portion of the baille below itsintersection with baille 13 extends downward and toward 4 the inlet endof the tank at an angle between about 30 and 60 from the vertical.

By following the above specifications, a tank of superior operatingcharacteristics is produced. The length of the tank provides greaterdistance for dotation to occur, and the depth of the tank, the locationofthe feed inlet and the location and design of the two bafflessubstantially triples the path of the uid in that the duid is directedback upon itself at a point about midway in the depth of the tank, andis then again reversed -to flow out below the bottomof bafHe 14. Thislong path provides ample opportunity for flotation and settling tooccur. The oil which is floated is carried to the point of skimming andconcentrated there, and the solids which drop to the bottom of the tankare swept to the lowest point for removal there. The lowest point in thetank may be located if desired below the' lower edge of baffle 14, or atany position behind the baffle as indicated.

As a specific example of a tank which is properly constructed, aflotation tank was built having a width W of about 2 feet, a liquiddepth DI at the inlet end of about 5 feet, (note that the tank wasconstructed with an adjustable weir 15 which permitted the control ofthe liquid level over a range of about 6 inches), and a length L, to thepoint of intersection of baflles 13 and 14, of about 9 feet. The top ofthe feed inlet was not more than 3 inches below the liquid level, thetop of the primary baille extended about 3 inches above liq-V uid level,the bottom of the bafHe 13, Where it joined bal-lie 14 was about 2.5feet below the liquid level, the radius of curvature of baffle 13 wasalso about 2.5 feet, and the center line of this radius of curvature wasabout 1.25 feet below the liquid level. The bottom of the secondarybaffle 14 was located at a depth of about 4.5 feet from the liquidlevel, which was approximately 0.75 times the depth of the liquid atthis point. The radius of curvature of bathe 14 was about 5 feet, andits center line was located at the liquid level. This made the averageslope of the lower portion of the baflle 14 about 45 from the vertical.Thus W equalled 0.4 DI or 0.2 L, DP equalled 0.5 DI, DC equalled 0.5 DP,and RP equalled 1.0 DP. Similarly D5 equalled about 0.75 Do, and RSequalled about 1.0 DI.

The above 'flotation unit was employed in a flotation operation asfollows. The waste water feed to the unit was a brine which hadoriginally been produced together with heavy crude California petroleum,and had been separated from the petroleum in a dehydrator and allowed tostratify for further separation in a tank and also in a skimming pond.The material still contained about 100 parts per million of oil however,which could not be separated by a further skimming operation. The wateralso contained about 100 parts per million of suspended solids. Thisfeed Water was not obviously emulsilied or particularly turbid, but wasentirely unsuitable for disposal into a sewage system, since thepermissible limits on such sewage were less than 25 parts per million.This feed was pumped through the above flotation unit at a rate of about5000 barrels per day. Since the effective volume of the tank (L x W x DIabove) was cubic feet or 16 barrels, this rate is equivalent to about310 volumes per volume per day, or 13 volumes per volume per hour, or aresidence time of less than 5 minutes. A centrifugal pump was employedto raise the pressure on the feed to about 40 to 45 pounds per squareinch gauge, and air was forced into the 4pressurized water from a sourcehaving a pressure of about 75 pounds per square inch gauge, at a rate ofabout 2.5 standard cubic feet of air per barrel of feed water and at apoint within about 2 feet of the pump discharge, The resulting aeratedwater was discharged through a valve near the inlet of the aboveflotation cell, which was operated at atmospheric pressure. The airreleased by the reduction in pressure arose through the liquid in theform of very tiny bubbles which formed a foam about an inch deep alongthe entire surface of the liquid in the tank. This foam was skimmed fromthe within the required limits.

For largerv installations, it has been found preferable to employ anumber of cells of approximately the above dimensions, rather than onesingle cell of correspondingly larger proportions. In one suchinstallation 6 cells are being operated continuously side by side inessentially thesame manner as above. In this case the feed wate'rcontained slightly more oil. In another operation, where it was desiredfor the purpose of avoiding corrosivity of theA feed water, to avoid theuse of air, natural gas was substituted for air in the above flotation.The flotation cell was enclosed in a gas-tight cover, and operated at apressurev slightly above atmospheric to avoid possible leakage of airinto the equipment, and the gas escaping fromthe flotation cell wasrecompressed and recycled to the feed. It was found that excellentresults were obtained, comparable to the use of air in the process.Other gases, such as nitrogen, hydrogen, and the like, may also beemployed in the process. lIn other operations, reduced pressures (inline 4) of about l5 pounds gauge have also been found suitable, and inone instance, a pressure not over 5 pounds gauge was operable.

It as also been found that in some cases it is desirable to allow acoagulationv period to follow the flotation biefore discharging thewater. Thus if a skimming pond, a settling tank, a second flotationunit, or even a pipeline of suicient size and length is interposedbetween the flotation cell discharge and the sewer or other disposal,further oil may agglomerate as a result of the flotation treatment andthis will still further purify the product. This coagulation is usuallypromoted by moderate agitation, but not by violent stirring. Acoagulation period of as little as one-quarter hour is usuallysutiicient, although longer periods obviousy do no harm. Theagglomerated oil may be filtered or skimmed from the so purified water.

In the above detailed description, no mention has been made of the useof chemicals in the process. It has been found however that there arewaste waters and other oilcontaminated waters which are not eicientlypurified simply by notation as above described. This isparticularly trueof waters which contain tight oil emulsions aud/or oil-wet solids orscales. These waters arealso frequently unaffected by the conventionalchemical treatment with oil dehydrating chemicals or iflotation agents,followed` by mere skimming operations; but where the above flotationmethod is employed in conjunction with certain additives, even the mostdiiii'cult Yof these waters has been purified. The chemical may be addedeither just before pressurizing the feed water', as through line 10a inthe drawing, or just after depressurizng the Water entering the tank, asthrough line 10 of the drawing. The additive is water-soluble, and ispreferably employed in aqueous solution, in amounts between about l and300 parts per million' and preferably between about l0 and 100 parts permillion of the additive.

Where oil-wet clays of water-dispersible type predominate, `highmolecular weight amine salts are preferred. Quaternary ammonium saltadditives having at least aegee?? about- 10 carbon atoms as hydrocarbongroups attached to the pentavalent nitrogen, such as trimethyloctadecylammonium chloride, methyl pyridinium chloride and hexyltri'ethylammoniumv acetate are effective, as well as other molecular weightrosin, alkyl and alkenyl amine salts such as rosin amine acetate,octadecylamine acetate, and oleyl amine hydrochloride.

Where solids are' no problem, but oil is present in a tight emulsion,high' molecular weight (greater than about" 120) amines and amitiee areeffective. Examples 6 of suitable amines are the heterocyclic aminessuch as' Nlphenyl niorpholine;v andthe substituted imidazole N-i--on Rtla.

where R is a hydrocarbon groupliaving'A about l0 to 20 carbonV atoms andR is an amine or alcohol group having no more than about 3 carbon atoms.A commercially available example isv the derivative in which R is koleyl(C17H33-) and R is hydroxyethyl -CHz-CHOHL Other examplesv areI thederivative in which R is lauryl (C12H25 iS the derivative in which R-islauryl and iRf's hydroxyl. Other effective' amines are those of? theformula where R and R have the above significance. Examples are thederivative in which R is oleyl and R is hydroxyethyl, the derivative inwhich R is lauryl and R is the ethylenediamine group, and the derivativein which R' is lauryl and R' is the aminomethyl group.

The amides which are also effective for this purpose have the formula inwhich R and R have lthe same significance as above;l Examples are (l)the derivative in' which R is lauryl and R is hydroxyethyl, (2) thederivative in which -R is oleyl and R is hydroxyl and` (3) thederivative in which R is palmityl aud R is aminomethyl.

Although the apparatus and process have been described ratherspecifically above, there are many modiications which are within thescope of the invention. The feed water, although generally containingless than 1% oil, mayy contain more oil, providing that the product isnot too viscous. For the proper application of flotation, the feedliquid must not be substantially more viscous than water itself. Theamount of oil which can be tolerated in lthe feed will therefore dependto some extent on the character o'f the oil emulsified or entraine'd inthe water, but in general the process is most suitable for aqueous feedscontaining not more than about 1000 parts per million of oil. The oilmay be a light distillate or a heavy residual oil or crude, or coal taror shale product or the like. The water may be a4 brine containingsubstantial amounts of dissolved salts, since these do not interferewith the process. l In fact, dissolved bicarbonates in amounts of about1% or less are actually beneficial. The insoluble solids content of thefeed liquid may be as high as 0.1% or higher.

The flotation gas is preferably air, particularly where the feedcontains dissolved or suspended iron compounds, although as indicatedabove other gases such as natural gas, hydrogen, etc., may be employed.The rate of feed nocaut? 1 'The pressure :of the .dotation operation isgenerally atmospheric, but higher-or lower pressures may'be entr-l I 1 yAploy'ed by enclosing the tank, as indicated in the'naturai i =gasexample given. above. l The! pressure; to .which lthe v water is raisedduring; solution of they flotation lgas is:l preferablyl between y2. to5 .times the pressure of the I flotation' tank, sol 'as tof provide iforAappreciable solution 3 ofthe gas and liberation in the tank; f,However, lower pressures arel operable. If .the tank is operated at at-Emosphericrr pressure. fori example (approximately l5 pounds per square;rinch absolute pressure) fand: .air l is 1 r employed .the otaton'gas,the .preferred pressureV m line 24'lies1 between about.y 1S andSO poundsper .square inch gauge, although pressure'from=about'131to.1about1= 100poundsl per square inch gauge would also be operable.

F. to zl-l20 F. have also been employed sucthe ota'tion apparatus 1 1vof said1 vessel at an 1angle of 30 lto' 60? .from the verticall 1 andterminating a minor distanceabovethe bottom of l l said. vessel;meanszfor withdrawing oated impuritiesfrom :above said battle; 1 and.means fory r*and process described above may also be r employed to `f l1 carry out chemical'reactions involving flotation. For example,- where:iron sulde'is to beconverted'toiron oxide,l 1 1 l and 'the -ironsulfideis in the form of a fine-suspension` r ofi solidi material vin weten:such a1 suspension may be employed as :the feed liquid,y `and: airA may;be 1employed l 1 i as the flotation gas. The iron suldeis 'thenconverted *to iron 'oxide and free sulfur in the. flotation operation,tank: 'and and-'the sulfur is floated Eto the' top 'cfs lthe skimmed offjust as in the case of the oil. 1 1

cur! yto one skilled. in rthe l art 1 fromlthe: above .descrip-- yWeclaim: f =1.- An apparatus for levelnear the top thereof, said cellbeing relatively nar'-y the: purification of' liquids 'by' f 1 dotation:which comprises aotationcellzwhich is-adapted- 1 to contain .a body ofliquid'r having a surface at :aliquid liquid from below said baffle. f 11 g 1 6, .A dotation ,cell .which iskr adapted to contain a body ofliquid having ;asurface; ata liquid Alevel near the top 1 1 thereof,said cell being relatively. narrow, tall and: long l; 1 1

:vided Afor withdrawing` solid impurities from the lowest1-pointoffsaidbottino.-.` 4. Auapparatusaccording to; claim ;l .in:which means. 3 l p 1 lare.provided Afor introducing 1chemicalsintoftheifeedr prior to.' its lintrruluctionfinto theflotaton; cell. 1

r5. rA flotation apparatus comprising means for` pres#c 1 1 surizing aliquid feed; means for introducing a dotation gas into: said pressurizedfeed; means for dischargingkr the pressurized mixture :at -a.reducedlpressure horizorrl tally .into the upper portion of the inlet end of anupright l dotation Avessel having major dimensions. of length and heightandi a minor dimension of width;- al substantially vertical bathedisposed near the outlet end of said vsel perpendicularto .and betweenthe side walls lof said vessei, said baille extending; downwardly ffrom.the upper 1 1 l .edge oisaid vessel to a lower region in said, vesselaudhavingits lower portiony ycurved toward the inietend i :and comprisinganinletend and .an outletfend oppol f site thereto,v -inlet rmeans; at;the inlet lend. .thereof just l below the leve-l of the liquid tobecontained'therein, a

' substantiallyvertcal primary baille neanthe outlet end 1 1'Other1modifications of this invention which 'would :oc-1

1 tion-thereof, are to be included within 'the scope' of the if 1invention' asdeined inthe following claims;

1 let endA opposite1 th'e1r'eto',a'y substantially vertical 'primary' vbaille near the `outlet end thereof, said primary baffle beingperpendicular to the side Walls of said flotation cell but having aconcave curvature facing the inlet end thereof and having its upper endabove the level of the liquid to be contained in said otation cell andits lower end extending approximately midway to the bottom of saidotation cell, a substantially vertical secondary baille joined atapproximately its midpoint to the bottom edge of said primary baille,said secondary baille also being perpendicular to the side walls of saidflotation cell but having an upper portion extending behind and abovethe primary baille so as to form an oil discharge trough therebetween,and having a lower portion which extends below the primary baille themajor portion of the remaining distance to the bottom of the dotationcell, said lower portion extending downward and toward the inlet end ofthe flotation cell at an angle between about 30 and 60, means formaintaining the desired liquid level within said otation cell, inletmeans for introducing the feed liquid into said flotation cell at apoint just below the desired liquid level at the inlet end thereof,means for skimming floating impurities from the liquid surface in saidotation cell over the top of said primary baille into said oil trough,means for withdrawing floated impurities from said oil trough, and meansfor withdrawing purified liquid from below said secondary baille.

2. An apparatus according to claim 1 which alsocom prises means forpressurizing said feed liquid, means for introducing a dotation gas intosaid pressurized feed liquid, means for reducing the pressure on theresulting mixture, and means for introducing the depressurized mixtureinto said inlet means. y

.3. An apparatus according to claim 1 inf which the bottom of thedotation cell is sloped, and means are prothereof, said primaryvlzlatlev having: aA concavel curvature 1 1 1 1 `facing the inlet end .ofsaid flotation .cell and-having its I, 1 1

upperr endabove .the surface ofthe liquid in said flota-l 1 tion celland? its lower endj extending approximately: mid. 1 way' to the bottomof saidotation cell, a Asubstantially1`1 i vertical secondary baillejoined at approximately its mid- 'point tor the `bottom .edge lof saidyprimaryl baille, js'aid f j 1 secondary baffle extending fbehindandabovetheprim'ary 1 1 1 1 bafiie so ,toy Aform1 an oil discharge ltroughltherebe- 1 -twcerunand having a. lower tportion'which iextendsbelow 1the primary bale the major portion; of the fremaining 1 l 1 .distancetothe bottomof the yflotation cell, said lower portion extendingdownward and toward the inlet end of the otation cell at an anglebetween about 30 and 60, and means for maintaining the desired liquidlevel within said flotation cell.

7. An apparatus according -to claim 6 in which the width of theflotation cell is between about 0.2 and 0.7 times the yliquid depth atthe inlet end, and is also between about 0.1 and 0.5 times the length ofthe cell from the inlet end to the bottom of the primary baffle, thebottom of the primary baille is at a depth between 0.3 and 0.7 times thedepth of the liquid at the inlet end of the cell, the radius ofcurvature of the primary baille is between about 0.7 and 1.5 times itsdepth, the center of the circle defining the radius of curvature of thisbaille is between about 0.3 and 0.7 times its depth, and the depth towhich the secondary baffle extends is between about 0.6 and 0.95 timesthe depth of the liquid at the location of the bottom of the bafiie.

8. An apparatus according to claim 6 in which the secondary baffle has acurvature concave toward the inlet end of the flotation cell, the radiusof curvature is between about 0.7 and 1.5 times the depth of the liquidat the cell inlet, and the center of the circle defining the radius ofcurvature is located at approximately the liquid level.

9. A method for treating an aqueous liquid to separate,

l0,carbon atoms as hydrocarbon groups attachedltothef` p'entavalentnitrogen, primary rosn, alkyl and alkenyl withdrawing .puried 1 1 1 f 9amine salts having at least 10 carbon atoms, substituted imidazo-leshaving the formula:

and amides having the formula:

o RCNR' in which R is a hydrocarbon group having about 10 to 20 carbonatoms and R' contains no more than 3 carbon atoms and is selected fromthe group consisting of amines and alcohols.

10. A method according to claim 9 in which the R is oleyl and R ishydroxyethyl.

11. The method according to claim 9 which comprises mixing saidilotation gas, chemical and liquid under superatmospheric pressure,discharging the resulting mixture horizontally into the upper portion ofthe inlet end of an elongated otation zone at lower pressure,withdrawing impurities from the surface of the liquid near the oppositeend of said otation zone, reversing the liquid 110W in the upper portionof the flotation zone near 10 said opposite end, and withdrawingpurified liquid from a lower portion of said opposite end of said zone.

12. A method according to claim 1l in which the feed liquid alsocontains solid impurities which are released by the flotation and arewithdrawn from the bottom of the otation zone.

References Cited in the le of this patent UNITED STATES PATENTS1,116,903 McClintock Nov. 10, 1914 1,376,459 Pederson May 3, 19211,505,944 Broadbridge et a1 Aug. 26, 1924 2,078,653 De Groote et al.Apr. 27, 1937 2,217,143 Stevenson Oct. 8, 1940 2,220,574 Little et al.Nov. 5, 1940 2,262,743 De Groote et a1 Nov. 11, 1941 2,330,589 QuellSept. 28, 1943 2,334,703 Henkel Nov. 23, 1943 2,382,490 Lawlor Apr. 14,1945 2,446,655 Lawrason Aug. 10, 1948 2,447,511 Leaf Aug. 24, 19482,641,571 Leutz et al June 9, 1953 2,759,607 Boyd et al. Aug. 21, 1956OTHER REFERENCES Chem., pages 126-130, vol. 35, January 1943.

9. A METHOD FOR TREATING AN AQUEOUS LIQUID TO SEPARATE OIL IMPURITIESCONTAINED THEREIN WHICH COMPRISES SUBJECTING SAID LIQUID TO CONTACT WITHA FLOTATION GAS IN THE PRESENCE OF A CHEMICAL COMPRISING AWATER-DISPERSIBLE AMINO COMPOUND SELECTED FROM THE CLASS CONSISTING OFN-PHENYLMORPHOLINE, QUATERNARY AMMONIUM SALTS HAVING AT LEAST 10 CARBONATOMS AS HYDROCARBON GROUPS ATTACHED TO THE PENTAVALENT NITROGEN,PRIMARY ROSIN, ALKYL AND ALKENYL